I would likely open the file for *reading* and search, using a while loop and
the match operator, for the 'email' record. If that exists then exit. Otherwise, seek to the beginning of the file, open a new file and write the contents of the existing file to the new file, adding the desired record at some point. How this is done would depend on the format of your file. (If it consists of lots of lines of text, then you can use the match operator to decide when to insert the desired record.) Finally, I'd
rename the old file (with a .bak suffix perhaps) and the newly
written file to "people.dat" or whatever.
(Opening the file in append mode will only allow you to add the new record at the end, but if that's what you want then you might do that. You won't be able to add a line in the middle of the file, though.)
(Update: Of course, do some torture tests with your code to make sure it works as desired before putting it into production.)

Ada Lovelace for the palindrome
Albert Einstein for having smelly feet
Alfred Nobel for his contribution to battlefield science
Burkhard Heim for providing the missing link between science and mysticism
Claude Shannnon for riding a unicycle at night at MIT
Donald Knuth for being such a great organist
Edward Teller for being the template for Dr. Strangelove
Edwin Hubble for pretending to be a pipe-smoking English gentleman
Erwin Schrödinger for cruelty to cats
Hedy Lamarr for weaponizing pianos
Hugh Everett for immortality, especially for cats
Isaac Newton for his occult studies
Kikunae Ikeda for discovering the secrets of soy sauce
Larry Wall for his website
Louis Camille Maillard for discovering why steaks taste good
Marie Curie for the shiny stuff
Nikola Tesla for the cool cars
Paul Dirac for speaking one word per hour when socializing
Richard Feynman for his bongo skills
Robert Oppenheimer for his in-depth knowledge of the Bhagavad Gita
Rusi P Taleyarkhan for Cold Fusion
Sigmund Freud for his Ménage ā trois
Theodor W Adorno for his contribution to the reception of jazz
Wilhelm Röntgen for the foundations of body scanners
Yulii Borisovich Khariton for the Tsar Bomba
Other (please explain why)